1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing a piston of an internal combustion engine and more particularly to a method and apparatus for manufacturing a lightweight piston, which is high in rigidity, of an internal combustion engine.
2. Description of the Related Art
A conventional method for manufacturing a piston of an internal combustion engine, as shown in FIG. 8, is disclosed in Japanese Utility Model Publication No. 3(1991)-26287. In the above prior method, holding dies 72,73,74 and a punching core 75 are set at a first step, so as to form a space 71 therebetween. At a second step, melted aluminum alloy 77 is poured into the space 71, and is pressurized so as to thereby urge the punching core 75 downwardly. At a third step, the holding dies 72,73,74 and the punching core 75 are removed and a crude piston is cast. At a fourth step, the crude piston is finished.
In general, the piston comprises a land portion, a skirt portion and a cavity. In order to assure the stable or reliable movement of the piston in the internal combustion engine, a lower portion of the skirt portion is thickened and rigidity thereof is increased. The punching core 75 has an incline surface to easily remove the punching core 75 from the holding dies 72,73,74. Namely, an outside diameter of the top portion of the punching core 75 is smaller than an outside diameter of the base portion thereof. Thus, an upper portion of the skirt portion has to be more thickened. As a result, the piston becomes heavy and the smooth movement of the piston may be injured.